The present invention relates to an optical element and an optical pickup device, and in particular, to an optical element that transmits a light flux coming from a light source with a short wavelength and to an optical pickup device employing the optical element.
In recent years, there have been advanced rapidly the research and development for the high density optical disc system that can conduct recording and reproducing of information by the use of a violet semiconductor laser with a wavelength of about 400 nm. As an example thereof, in the case of the optical disc conducting recording and reproducing of information under the specifications of NA 0.65 and light source wavelength 407 nm, information of 20-30 GB per one side can be recorded for the optical disc with a diameter of 12 cm that is the same as that of DVD (NA: 0.6, wavelength of light source: 650 nm and storage capacity: 4.7 GB).
(Patent Document 1) TOKKAI No. 2003-303436
(Patent Document 2) TOKKAIHEI No. 8-179103
Incidentally, in the optical pickup device, there is used an optical element made of plastic and is relatively inexpensive and can be manufactured easily on a mass production basis, and on the optical surface of the optical element, there is formed an antireflection film. However, according to the tests made by the inventors of the present invention, it was found that, when a laser beam with a short wavelength emitted from the violet semiconductor laser was applied to the optical lens made of plastic for a certain period of time, optical characteristics of the irradiated optical element were deteriorated considerably. Incidentally, in the Patent Document 2, there is disclosed a technology for forming a film of a material MgFx (x=2-2.1) on a base board. In this case, however, optical characteristics are not deteriorated even if the laser beam with a short wavelength is applied, because the base board is made of glass.